Game Development by Sean

Compile-time String Hashing in C++0x

I have written a small demo (tested in GCC 4.6) showing how to get compile-time hashing of a string literal in C++0x.

(Note that the blog is having a little bit of fun with the formatting of the code below; my apologies.)

#include

// NOTE: hashing algorithm used is FNV-1a

// FNV-1a constants static constexpr unsigned long long basis = 14695981039346656037ULL; static constexpr unsigned long long prime = 1099511628211ULL;

// compile-time hash helper function constexpr unsigned long long hash_one(char c, const char* remain, unsigned long long value) { return c == 0 ? value : hash_one(remain[0], remain + 1, (value ^ c) * prime); }

// compile-time hash constexpr unsigned long long hash(const char* str) { return hash_one(str[0], str + 1, basis); }

// run-time hash unsigned long long hash_rt(const char* str) { unsigned long long hash = basis; while (*str != 0) { hash ^= str[0]; hash *= prime; ++str; } return hash; }

// ensure that the constexpr value is really compile-time constant template struct test_const { static const unsigned long long value = NUM; };

// test int main() { constexpr unsigned long long value = test_const<hash (“hello, world!”)>::value; unsigned long long value_rt = hash_rt(“hello, world!”);

std::cout < < "hash =    " << value << std::endl;
std::cout << "hash_rt = " << value_rt << std::endl;

return value == value_rt ? 0 : 127; } </code>

The code is available at GitHub as well, as will be any future updates/tweaks I make for the demo.</hash></unsigned></iostream></code></div>